Thermal flowmeter and flow rate correction method

Abstract

A thermal flowmeter includes a first thermal resistance element that detects a first temperature of a fluid, a second thermal resistance element disposed downstream to detect a second temperature of the fluid, a control unit that causes the second thermal resistance element to heat to make the second temperature higher than the first temperature by a fixed value, a power measurement unit that measures a power applied to the second thermal resistance element, a power conversion unit that multiplies the power measured by the power measurement unit, by a constant uniquely determined depending on the fluid, thereby converting the power to a power required when the fluid is water, and a flow rate calculation unit that calculates a flow rate of the fluid, by converting the power converted by the power conversion unit to a value of the flow rate, using a flow rate conversion characteristic formula applicable to water.

What is claimed is: 
     
         1 . A thermal flowmeter comprising:
 a pipe through which a fluid to be measured is caused to flow;   a first thermal resistance element disposed on the pipe and configured to detect a first temperature of the fluid to be measured;   a second thermal resistance element disposed on the pipe at a position downstream of the first thermal resistance element and configured to detect a second temperature of the fluid to be measured;   a control unit configured to cause the second thermal resistance element to heat by outputting a voltage to make the second temperature higher than the first temperature by a fixed value;   a power measurement unit configured to measure a power to be applied to the second thermal resistance element;   a power conversion unit configured to convert the power measured by the power measurement unit to a power assumed to be required when the fluid is water, by multiplying the power measured by the power measurement unit by a constant uniquely determined depending on a type of the fluid to be measured; and   a flow rate calculation unit configured to calculate a flow rate of the fluid to be measured, by converting the power converted by the power conversion unit to a value of the flow rate, using a flow rate conversion characteristic formula applicable when the fluid is water.   
     
     
         2 . The thermal flowmeter according to  claim 1 , wherein
 the constant is determined through an experiment performed beforehand, on a basis of a power obtained by backward calculation based on an inverse function of the flow rate conversion characteristic formula, from an actual flow rate of the fluid to be measured and a flow rate measured by the thermal flowmeter.   
     
     
         3 . A flow rate correction method for a thermal flowmeter including a pipe through which a fluid to be measured is caused to flow, a first thermal resistance element disposed on the pipe and configured to detect a first temperature of the fluid to be measured, and a second thermal resistance element disposed on the pipe at a position downstream of the first thermal resistance element and configured to detect a second temperature of the fluid to be measured, the flow rate correction method comprising:
 causing the second thermal resistance element to heat by outputting a voltage to make the second temperature higher than the first temperature by a fixed value;   measuring a power to be applied to the second thermal resistance element;   converting the power measured in the measuring of the power to a power assumed to be required when the fluid is water, by multiplying the power measured in the measuring the power by a constant uniquely determined depending on a type of the fluid to be measured; and   calculating a flow rate of the fluid to be measured, by converting the power converted in the converting the power to a value of the flow rate, using a flow rate conversion characteristic formula applicable when the fluid is water.   
     
     
         4 . The method according to  claim 3 , wherein
 the constant is determined through an experiment performed beforehand, on a basis of a power obtained by backward calculation based on an inverse function of the flow rate conversion characteristic formula, from an actual flow rate of the fluid to be measured and a flow rate measured by the thermal flowmeter.